High-payload, high-precision, six-degree-of-freedom, positioning systems are useful for diverse tasks ranging from sub-micron manipulation of semiconductor photo-masks during electron beam patterning to active position adjustment of multi-ton jet engines during aircraft manufacturing.
One of the most widely used positioning systems is the strut array. Strut arrays belong to a class of manipulators known as Stewart mechanisms and are versatile and inexpensive. When properly arranged, they have the advantage of being kinematic, meaning that they allow the position of an object being supported to be adjusted in all six degrees of freedom (x, y, z, pitch, roll and yaw) without over constraining the object. Unlike positioning mechanisms assembled from linear stages, strut arrays are not orthogonal: pure translations require that the lengths of all of the struts be adjusted. If absolute alignment tolerances are relatively loose, this actuator coupling, manifested as small cosine errors, is not generally significant. However, strut arrays are often limited in translational range of motion.
A new system called a “tri-sphere” was introduced recently [See “An Automated Magnet Positioning System for Use in the Next Linear Collider”, R. J. Viola, Department of Energy Report No. DOE-ER84078-SQR1, incorporated herein by reference.] In a trisphere system, three jacks provide support for an object being manipulated. Each jack is adjustable in two out of three orthogonal (or nearly orthogonal) directions and free in the third direction. For example, a system may be constructed in which three jacks are arranged in a triangle. Each jack includes vertical (z-direction) adjustment. Successive jacks are adjustable in one lateral (x or y) direction and free in the other and one of the jacks is rotated 90 degrees relative to its neighbors according to Table 1.
TABLE 1Orientation of actuated and free (unconstrained) axes in a tri-spheresystem.Jack #x-movementy-movementOrientation relative to Jack #11FreeActuated—2ActuatedFreeRotated 90 degrees3FreeActuatedParallel
A tri-sphere system was created from commercial motion control components. Each jack mechanism comprised a traveling block, riding on a pair of linear bushings, driven in the horizontal plane by a motorized lead screw. A central ball screw, driven by a geared motor connected via a spline shaft, provided vertical adjustment. The central ball screw was topped by a steel contact sphere (hence the name “tri-sphere”) that acted as the interface between the mechanism and the object being manipulated. The spheres engaged V-shaped grooves incorporated into the bottom of the object. The grooves were located at right angles to the lead screws that drove the traveling blocks. Because of the design's inherent compliance, the object did not have to be precisely located relative to the three support points when being installed. When lowered into a nominally correct location the object was snapped into place by gravity.
While quite successful for some applications, the conventional tri-sphere has limitations that render it unsuitable for other applications. For example, the conventional tri-sphere can only be used upright. In other words the jacks must support the object from the bottom. If the system were turned upside down the object would fall off. There are many applications such as manipulation of objects in a cryostat in which the object must be suspended from its manipulator. Unfortunately the conventional tri-sphere cannot be used in suspension mode.
The conventional tri-sphere is not compatible with high-vacuum and/or low-temperature scientific apparatus partially because its actuators are unusable in those situations. Ball screws and electric motors, for example, are not compatible with high-vacuum, low-temperature chambers.
Finally, there are several applications of positioning systems in which an object is initially positioned and subsequently not often adjusted or perhaps never adjusted again. The conventional tri-sphere may be used in these applications but it is needlessly expensive to employ precision actuator motors for only a single use.
What is needed is a six-degree-of-freedom positioning system that has the advantages of the tri-sphere yet is compatible with upside down, inverted or suspended orientations. Further, what is needed is an inverted positioning system that is compatible with high-vacuum and/or low-temperature apparatus. Finally, what is needed is a positioning system that includes the advantages of precise motor control without wasting expensive motors in single- or low-use applications.